Laser processing is employed on a variety of electronic devices to achieve a variety of effects. Typically electronic devices appear at various stages of their manufacture as substantially planar arrays of identical components referred to as workpieces. Examples of workpieces include semiconductor wafers, printed or etched wiring or circuit boards, or arrays of passive or active components built upon ceramic or silicon substrates, among others. In general, workpieces are conveyed to and from the particular apparatus performing the laser processing either individually or in batches, often being carried in cassettes or magazines that can be automatically unloaded and loaded. The term laser processing includes loading the workpiece onto the apparatus, aligning the workpiece to the apparatus, performing the laser processing, and then unloading the workpiece from the apparatus. Laser processing can be conducted on numerous different workpieces using various lasers effecting a variety of processes. Examples of laser processing include laser processing of a single or multilayer workpiece to effect hole and/or via formation and laser processing of a semiconductor wafer to effect wafer dicing or singulation. The laser processing methods described herein could also be applied to many other types of laser material interaction processes, including but not limited to removal of semiconductor links (fuses), thermal annealing, trimming passive components, or scribing or singulating wafers, including silicon, or substrates, including ceramic.
Several factors determine the desirability of a laser processing apparatus. These include accuracy, quality, usability, flexibility and throughput. These also include such apparatuses having multifunctional capability. Adding other functions to the apparatus allows the user of such apparatus to either gain throughput by avoiding transferring the workpiece between differing apparatuses, reducing cost through the elimination of differing apparatuses, or both. Throughput is a very important consideration because of its direct impact on the cost of processing on a per workpiece basis. System throughput is a function of several factors, including material removal rate, workpiece and laser beam positioning speed and other system overhead. System overhead is the time for all operations of a laser processing apparatus not directly involved with material removal or modification. It may include loading and unloading workpieces, aligning workpieces, inspecting workpieces, waiting for mechanical components to settle following motion, and waiting for lasers and other electronic components to settle electrically upon powering up or changing parameters.
Material removal rates for via formation in multilayer substrates by laser processing is partially determined by the complexity of the multilayer substrate being processed which is a function of factors generally beyond the control of the laser system designer. FIG. 1 shows an exemplary multilayer workpiece 10 of an arbitrary type that includes layers 12, 14, 16, and 18. Typically, layers 12 and 14 are metal layers that each include aluminum, copper, gold, molybdenum, nickel, palladium, platinum, silver, titanium, tungsten, a metal nitride, or a combination thereof. Metal layers 12 and 14 may have thicknesses that are between about 9 μm and about 36 μm, but they may be thinner than 9 μm or as thick as 72 μm or more.
Each layer 16 typically includes a standard organic dielectric material such as benzocyclobutane (BCB), bismaleimide triazine (BT), cardboard, a cyanate ester, an epoxy, a phenolic, a polyimide, polytetrafluorethylene (PTFE), a polymer alloy, or a combination thereof. Each organic dielectric layer 16 is typically thicker than metal layers 12 and 14. The thickness of organic dielectric layer 16 may be between about 30 μm and about 1600 μm.
Organic dielectric layer 16 may include a thin reinforcement component layer 18. Reinforcement component layer 18 may include fiber matte or dispersed particles of, for example, aramid fibers, ceramics, or glass that have been woven or dispersed into organic dielectric layer 16. Reinforcement component layer 18 is typically much thinner than organic dielectric layer 16 and may have a thickness that is between about 1 μm and about 10 μm. Reinforcement material may also be introduced as a powder into organic dielectric layer 16. Reinforcement component layer 18 including this powdery reinforcement material may be noncontiguous and nonuniform.
Layers 12, 14, 16, and 18 may be internally noncontiguous, non-uniform, and non-level. Stacks having several layers of metal, organic dielectric, and reinforcement component materials may have a total thickness that is greater than 2 mm. Although the arbitrary workpiece 10 shown as an example in FIG. 1 has five layers, the present invention can be practiced on a workpiece having any desired number of layers, including a single layer substrate.
Material removal rate for a laser processing apparatus is also limited by the per-pulse laser energy available and pulse repetition rate. Increased processing throughput can be accomplished by increasing the pulse repetition rate at pulse energy sufficient to cause material removal via either ablation, thermal vaporization, or a combination of both. For most lasers used in processing applications, however, pulse energy is approximately inversely proportional to pulse repetition rate. As a result there will be a maximum rate of material removal governed by the minimum pulse energy needed to cause material removal and the maximum pulse repetition rate at which that energy is available. Selection of lasers, in terms of pulse energy available and pulse rate, is affected by technological advancement, cost, and other performance parameters which may limit the laser processing system designer's choice.
Another factor affecting system throughput of a laser processing apparatus is laser beam positioning speed. Laser processing typically involves directing a laser beam at a particular point on a workpiece and operating the laser for a specific duration or number of laser pulses. The laser beam is directed at the specific point on the workpiece by moving the workpiece, the laser beam or a combination of both. The laser beam can be directed to a specific location on the workpiece where laser processing is accomplished and subsequently directed to a next location where further processing is accomplished. Alternatively, the laser beam may be directed to move substantially continuously with respect to the workpiece, the laser beam then describing a path on the workpiece along which processing is accomplished by pulsing or otherwise operating the laser during the relative motion between the laser beam and the workpiece. Laser processing can also be accomplished by a combination of these methods. What is common to both of these methods is that the rate of material removal is influenced by the rate at which the laser beam's position with respect to the workpiece can be changed. Several factors influence the choice of motion control components that determine the speed of laser beam positioning including cost, accuracy, power consumption and size.
It is also necessary for laser power to be stable during processing to insure consistent, repeatable results. To accomplish this, apparatuses typically use laser power or energy detectors during processing to monitor laser power and verify that lasers are operating within necessary parameters. Both the lasers and the power/energy detectors contribute to system overhead because they need time to stabilize after being turned on, thereby decreasing system throughput each time they are turned on. Lasers are expensive components with useful lifetimes that are relatively limited and proportional to the length of time they are turned on. Thus, lasers are typically turned off if they are to be idle for prolonged periods during system operations such as loading and unloading workpieces.
System throughput can also be influenced by system overhead. This includes time required to load, align and unload workpieces. FIG. 2 shows a timing diagram for a prior art apparatus that loads, processes and unloads workpieces sequentially. Examination of the timing chart shown in FIG. 2 reveals that a substantial amount of the total time required to process a workpiece, shown in the diagram as time 0 to t2, on a laser processing apparatus is spent on the overhead activities of loading, aligning, and waiting for the laser to settle 20 (time 0 to t1) in relation to the time spent processing 22 (time t2-t1).
Some laser processing systems apply more than one laser beam to process more than one location simultaneously. An example of a prior art apparatus employing two lasers is the apparatus described in US Patent Application Publication 2005/00985496, “Laser Beam Machining Apparatus.” The apparatus disclosed therein has two workpieces mounted on the apparatus and processed simultaneously with two laser beams. FIG. 3 shows a timing diagram of this prior art approach to increasing throughput by processing two workpieces simultaneously. The two timelines labeled WP1 and WP2 denote processing being applied to two workpieces at the same time. Both workpieces are loaded during time periods 30 and 32. Both workpieces are processed during time periods 34 and 36. During time periods 38 and 40 both workpieces are unloaded and new workpieces are loaded into the apparatus. Processing on the two new workpieces occurs during time periods 42 and 44. Although this apparatus can yield up to twice the throughput of a single station prior art apparatus, examination of the timing diagram in FIG. 3 still shows substantial system overhead devoted to loading and unloading workpieces. During this load and unload time the lasers are not processing workpieces and are typically turned off. This approach suffers from the increased cost and complexity of adding an additional laser and the optical and mechanical components required to direct the laser beam to the workpiece, but still does not avoid the processing time delays associated with turning on and stabilizing the lasers.
There is a continuing need for an apparatus for performing laser processing of electronic components, capable of increased throughput when using either a single or multiple laser beams to process workpieces by improving the utilization of the laser and optical components.